The polysaccharide chitin is a structural cell wall component of most fungi and is the most abundant organic skeletal component of invertebrates, making up, for example, from about 25 to 60% of the dry weight of insect cuticles. Chitin consists primarily of linear polymers of the amino sugar N-acetyl-D-glucosamine joined in 1,4-.beta.-glucosidic linkage. Thus, chitin bears a close resemblance to cellulose, the major structural polysaccharide of plants, the only chemical difference being that in chitin the hydroxyl group on the 2-position is an acetoamido group instead of an hydroxyl group. However, because of its widespread occurrence in fungi and arthropods, the total world-wide production of chitin vastly exceeds cellulose.
Many fungi and arthropods having chitinous cell walls or exoskeletons are injurious to plants and animals, causing a legion number of diseases including, but not limited to, wheat eyespot, rice sheath blight, damping off, apple scab, pepper botrytis, rice blast, sugar beet cercospora, tomato early blight, wheat leaf rust, and wheat powdery mildew. Fungal species also cause a myriad of cutaneous and systemic mycoses in human beings and other animals, including, but not limited to, candidiasis, histoplasmosis, blastomycosis, pneumocystis, sporotrichosis and cryptococcosis. Insects can act as vectors of viruses causing arboviral encephalitides, yellow fever, and dengue, protozoa causing malarias, trypanosomiases, and leishmaniases, and various harmful helminths. Crustaceans also carry some infectious helminths and trematodes.
Most fungicides and insecticides that are used to control or cure these diseases by killing or controlling their causative agents, intermediate hosts, or vectors employ various modes of action including physical poisons that suffocate or desiccate organisms; protoplasmic poisons such as arsenicals that kill by precipitating or deactivating proteins, enzymes or other cellular constituents; respiratory poisons that deactivate respiratory enzymes; and various poisons that affect different tissue systems such as tubules or nerves. Of course, preferred agents do not injure the host plant or animal, and most preferably have no effect whatsoever on the host. Because of the complexity and interdependence of life processes, however,, this goal is not always achieved, so that many fungicides and insecticides exhibit some toxicity to the host. Others cause unexpected side effects.
Since chitin is not a usual constituent of most plants and vertebrates, chitin biosynthesis inhibitors can be employed as selective antifungal and/or insecticide agents. Applied to ornamental or edible plants or animals, these offer the advantage of targeting undesirable fungi or insects without harming significantly the host plant or vertebrate animal. While much attention has been paid to chitin synthesis, there have been very limited studies targeting screens which exploit chitin degradation. 1-(2,6-Dichlorobenzoyl)-3-(3,4-dichlorophenyl) urea, for example, has been suggested as a chitin-inhibiting insecticide. Antifungals that have also been found to inhibit chitin synthesis include nikkomycin and polyoxin D.